The discovery that angiotensin II (Ang II) is present in central nervous system (CNS) neurons and pathways which are believed to be involved in cardiovascular (CV), fluid, and electrolyte regulation suggests that the peptide may function as a neurotransmitter (NT) or neuromodulator, as well as a circulating hormone. However, in addition to localization within the CNS, for Ang II to be recognized as a NT the following criteria must be met: 1) Release in response to physiological stimuli; 2) Identification of receptors in suitable target neurons; and, 3) Demonstration of physiologically significant responses. The objective of this proposal is to address two of the above criteria by: 1) demonstrating the anatomical origin and pharmacological characteristics of saturable, high affinity Ang II binding sites found in the dorsomedial medulla oblongata (DMM) and the intra- and extracranial segments of the vagus nerves (No. 2 above); and, 2) relating these findings to the production of CV responses following injection of Ang II into nuclei of the DMM at physiologically relevant doses (No. 3 above). By the use of the sensitive, quantitative "in vitro" autoradiography technique, characteristics of Ang II binding in the DMM and vagus nerve of dogs will be determined in terms of density, saturability, affinity, and ligand selectivity before and following vagotomy, nodose ganglionectomy, and transection of the rostral solitary tract in the medulla. The anatomical relationship of Ang II binding to neurons and pathways in the DMM will be examined initially at the light microscopic level by correlating: 1) the distribution of Ang II binding sites with cytoarchitectural subdivisions of the canine nucleus tractus solitarii, area postrema, and dorsal motor nucleus of the vagus; and, 2) the pattern of Ang II binding sites to neurons and fibers containing substance P, or synthetic enzymes such as tyroxine hydroxylase. The relationship between changes in Ang II binding following sino-vagal deafferentation or de- efferentation of the DMM and alterations in hemodynamic responses produced by microinjection of femtomole doses either Ang II or Ang II antagonists into these areas will be evaluated in both anesthetized and unanesthetized animals. Changes in density and affinity of Ang II binding sites in the DMM and hypothalamic- hypophysial region will be examined following maneuvers known to result in chronic alterations in cerebrospinal fluid or plasma Ang II. These studies will provide essential information about the contribution of specific DMM neurons and pathways to Ang II binding sites in this region, the relationship of binding with NT- identified systems, and factors which regulate the binding sites. Finally, microinjection studies will provide insights into Ang II's role in modulation of CV function and interaction with reflex regulation of blood pressure.